Current-limiting fuse



Sept. 24, 19460 CIIGL sun's CURRENT-LIMITING FUSE Filed Nov. 11, 1942 m u P r km 9 0 ne e c A K s .a

Patented Sept. 24, 1946 2,403,351 CURRENT-LIMITING FUSE Chauncey G. Suits, Schenectady, to General Electric Company,

New York N. Y., assignor a. corporation of Application November 11, 1942, Serial No. 465,205

1 Claim. 1

My invention relates to fuses and more partic-' ularly tofuses of the current-limiting type.

Current-limiting fuses are known which consist in the main of a fusible element in the form of one or more conductors of suitable metal such as silver or zinc embedded in an inert granular or refractory arc-extinguishing material of high dielectric strength such as sand or relatively finely divided quartz, the conductor or conductors in some instances being wound on a supporting core of high temperature resistant ceramic material. To these parts are added an enclosing housing and terminals for the conductors.

Upon subjection of such fuse to currents of large magnitude, such as upon short circuit, the fusible element attains fusing temperature and dissipates or vaporizes throughout a fusible section of considerable length as distinguished from dissipation only at a point of reduced cross-section, whereupon arcing occurs and the metal vapors rapidly expand to many times the volume originally occupied by the fusible element itself.

The force of this expansion throws the metal vapors out of the arc path and into the spaces between the granules of inert filler material. These vapors condense upon the surfaces of the relatively cool granules and no longer are available for current conduction so that arcing is contact between the hot arc column and the relatively cool granules causes an exceedingly rapid transfer of heat from the arc to the granules resulting in a very high voltage gradient or a high voltage drop per unit length of arc column. The arc energy is dissipated in this heat transfer with practically negligible net pressure within the fuse housing. During the arcing period the granules in the vicinity of the arc melt and upon cooling form a hollow shell or fulgurite consisting of fused quartz if the arc-extinguishing material is granular quartz and a small percentage of metal from which the fusible conductor is made. This fused shell has the properties of an insulator and in the normal operation of the fuse does not constitute a current-conducting element in the current path.

As a result of the actions referred to in the preceding paragraph there is in effect suddenly inserted into the path of the current at the instant of melting of the fusible element, the high resistance formed by the arc column which initially limits the current to a magnitude which is only a small fraction of the current which the system with which the current-limiting fuse is associated is capable of producing. The insert. ed resistance causes rapid current decay and subsequent interrupting of the current flow with negligible mechanical disturbance there being no gassing or discharge of vapors or particles and practically no noise.

It is an object of my invention to provide a new and improved current-limiting fuse of the type escribed above.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claim annexed to and forming a part of this specification.

Referring now to the drawing, I have illustrated in a single figure a cross-sectional view of a currentlimiting fuse embodying my invention.

As was mentioned above, the fused shell which forms upon operation of a current-limiting fuse of the type referred to above is a relatively good insulator and although it is shunted across the terminals of the circuit to be interrupted, it does not under ordinary conditions limit the interrupting ability seriously. However, near the maximum voltage condition, under which the fuse will operate and when relatively minor impurities are found in the otherwise pure inert granular material such as quartz sand, the current conduction in this fused column does lead to interruption failures. I have found, however, that when a very fine grain crystalline quartz powder such as will pass through a 200-mesh sieve is used instead of the coarser quartz sand, the fulgurite formation is avoided, the fusion of the powder forming globules or beads instead of a continuous tube, as described above. I have found also that the voltage gradient, or volts per centimeter of length, is considerably increased with the fine quartz powder due to the increased cooling action on the arc column as compared with relatively coarse grain quartz sand such, for example, as -mesh sand.

The interrupting action and the current-limiting action of the current-limiting type of fuse are greatly improved when an intimate mixture of relatively coarse grain and relatively fine grain inert arc-extinguishing material is used as the arc-quenching medium, and I have found that particularly satisfactory results are obtained with a mixture of 80-mesh and ZOO-mesh granular material such as fused quartz, silica or the like. The fine grain material greatly increases the voltage gradient to produce a much higher currentwhile the coarse grain material tends to provide a more gradual insertion of resistance into the circuit so as to some extent minimize the voltage surges that are produced. By varying the proportions of the diiferent grain size materials which are intimately mixed, various operating conditions can be obtained although I have found an intimate mixture of 80- mesh and 200-mesh inert granular arc-extinguishing material most satisfactory from the standpoint of high current-limiting action without introducing too great a voltage surge.

Referring now to the drawing, I have disclosed a tubular enclosing casing which is constructed of glass, porcelain or other suitable insulating material. Metallic terminal ferrules H are secured respectively on the opposite ends of the casing :by means of any suitable form of glass-tometal seal indicated at l2. The ends of ferrules II are closed by caps l3 sealed in place although, if desired, the caps may be made integrally with the ferrules. An elongated insulating support or core l4 comprised of high heat-resisting ceramic or other suitable material extends longitudinally in casing ID. This .core serves to support a current-interrupting element or conductorl5 adapted to fuse throughout a section of considerable length, which is spirally wound on the core 14. Although the current-interrupting element H: of the illustrated device comprises only a single conductor, it is common in the construction of such current-limiting devices to employ one or more main conductors depending upon the current rating. This is for the reason that it has been found that most effective current-limiting action is produced when conductors of very thin cross-section are employed. Hence, where the current rating demands a relatively large cross-section, it is desirable to divide this total cross-section into a number of parallel-connected conductors having a thin cross-section such as a plurality of fine silver or zinc wires.

The interior of casing H], in accordance with my invention, is filled with an intimate mixture of relatively coarse grain and relatively fine grain inert granular arc-quenching material. The relatively coarse grain material which might be 80- mesh fused quartz or silica, is indicated at US while the relatively fine grain material which is preferably 200-mesh fused quartz, or silica, is indicated at [1. The intimate mixture of this material produces an improved current-limiting action without subjecting the associated circuits to abnormally high voltage surges. This intimate limiting action mixture of inert granular material indicated at I6 and I1 embeds the current-interrupting element l5. As will be obvious from the drawing, the core M is of generally star-shaped construction in that it has a, plurality of longitudinally extending fins or ribs, one of which is designated as l8. Although the illustrated core M has four such fins, a core having more than four fins may be employed depending upon the rating and size of the particular fuse. The advantage of the starshaped core is that the current-interrupting element IE or fusible element, touches the core only at spaced points so that the maximum length of the fusible element is exposed to the inert granular filler material indicated at It and I1. spirally winding the interrupting element on a core permits the installation of an element of relatively great length in a casing of practical dimensions.

The opposite ends of the fusible element iii are electrically connected with the terminals or ferrules H as by conducting means is which also serve to support the core M at the axis of tubular casing In.

It should .be understood that means to further limit the surge voltage such as are disclosed in Lohausen Patent 2,157,907, granted May 9, 1939, and assigned to the assignee of the present application, may be employed, although with my improved fuse construction such means for limiting the surge voltage may not be necessary.

While I have disclosed a particular embodiment of my invention, it will be apparent to those skilled in the art that my invention is not limited to the particular construction shown but that changes and modifications may be made without departing from the spirit and scope of my invention and I aim in the appended claim to cover all such changes and modifications.

What I claim as new and desire to secure by Letters Patent of the United States, is:

In a high tension fuse of the current-limiting type, the combination of a totally enclosed casing containing a body of arc-quenching medium mixture of 200-mesh and comprising an intimate -mesh granular inert material, and a conductor embedded in said material which conductor is adapted to dissipate throughout a fusible section of considerable length as distinguished from dissipation only at a, point of reduced cross-section upon subjection of the fusible section to excess of large magnitude.

CHAUNCEY G. SUITS.

current 

